Zebras may have stripes because horseflies hate them

Zebras are best known for their dramatic stripes, but why they evolved remains uncertain. One popular notion is that stripes make it difficult for predators to single out an individual zebra from the herd, but experimental evidence for this or other ideas has been lacking.

Zebras are best known for their dramatic stripes, but why they evolved remains uncertain. One popular notion is that stripes make it difficult for predators to single out an individual zebra from the herd, but experimental evidence for this or other ideas has been lacking.

Scientists investigating horses found that dark-coated individuals attracted far more bloodsucking horseflies than white ones did. Since zebra embryos start out with a dark skin but go on to develop white stripes before birth, the researchers wondered if their striped hides might help make zebras less attractive to the pests. Horseflies, also known as tabanids, not only deliver nasty bites, they also carry dangerous germs.

To see how unappealing zebra stripes might be, the researchers visited a horsefly-infested horse farm near Budapest. Tests with surfaces painted with black and white stripes of varying widths and angles and covered in glue revealed that the narrowest stripes drew the fewest insects, and that real zebra hides closely matched the patterns that were least attractive to tabanids.

The scientists also created all-white, all-black and black-and-white-striped horse models, expecting the striped one would attract a number of flies that was intermediate between the white and dark models. Surprisingly, the horseflies seemed to find the striped model the least appealing of all.

To understand why zebra stripes might have this effect, one can think of all light waves as electric fields rippling either up and down, left and right, or at any angle in between, a property known as polarization. Many insects are drawn to horizontally polarized light because it is a telltale sign of water. Light reflected off water is horizontally polarized; horseflies develop in water or mud, and so are drawn to stretches of water where they can mate and lay eggs. In contrast, the dark and light stripes of zebras each reflect different polarizations of light, and the fact they are arranged vertically might ward off horseflies looking for smooth, horizontally polarized signals, explained researcher Susanne Åkesson, an evolutionary ecologist at Lund University in Sweden.

The researchers do not exclude the possibility that zebra stripes might have benefits other than pest control. Still, "we believe that escaping biting flies, which are annoying to their hosts and transmit lethal diseases, would be a very important selection factor, which may have a much stronger effect than the benefits of striped coat patterns suggested previously," Åkesson told LiveScience.

The scientists do note that they have not yet performed these experiments in Africa, where the zebras live, but would expect similar results if they did. They also wonder if odors or other molecules from the zebras might attract horseflies, overwhelming any benefit the stripes might have — future models could include such chemicals, Åkesson said.

Åkesson, with Gábor Horváth and their colleagues, detailed their findings Feb. 9 in the Journal of Experimental Biology.